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 Pond Parasitic Disease Control with DTHP


By Bob Fenner

Aquatic Gardens

Ponds, Streams, Waterfalls & Fountains:
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by Robert (Bob) Fenner

Many of the visibly large external parasites of goldfish, koi and other Pondfishes, such as the crustaceans Anchorworm (Lernaea) and fish lice (Argulus), among others not so easily seen; flukes, some Protozoans are eradicated effectively using DTHP. When used appropriately; that is, occasionally in a correct dosage and application interval, it is a very useful, safe cure.

O,O-dimethyl 2,2,2-trichloro-1-hydroxyethyl phosphonate (DTHP) is the International Union of Pure and Applied Chemistry's

scientific name for an organophosphate insecticide that goes by many different trade names. Most people know it as Dylox, D50, Dipterex, Masoten, Trichlorofon, Foschlor or Neguvon. Yes, these are all names for exactly the same molecule. It is a white, dull powder.

There are many advantages of using DTHP:

1) It is readily water-soluble.

2) DTHP degrades within a period of hours to days in water

3) It will not interfere with biological filtration.

4) It will not discolor your water.

5) Is easily removed with activated carbon.

6) Is inexpensive and readily available.

7) DTHP has a wide range of safety.

8) Is relatively non-toxic to non-invertebrates.

External Parasites Susceptible To Treatment With DTHP:

Protozoans; Ichthyophthirius, Trichodina

"Worms"; Flukes ( e.g. Benedenia, Dactylogyrus, Gyrodactylus)

Leeches; e.g. Piscicola, Placobdella

Crustaceans e.g. fish lice (Argulus), anchor worm (Lernaea)

Snails and Insects as well (it is an insecticide)

Some Cautionary Remarks:

As well-detailed in many other offerings of this type; it is extremely valuable to understand as much of "the big picture" when undertaking the treatment of any "diseased condition". Environmental, nutritional, water quality influences should be sized up before undertaking a chemical/physical war.

Towards the ends of "arming" oneself, it is suggested that you discuss your situation with local merchants, hobbyist groups, university representatives, and/or a library of some stature before actually committing yourself to a given regimen. There are, indeed, several interacting sets of factors to contend with. Some discussion of these follows. It is presumed at this point that the Reader is familiar with the optimal and tolerable living conditions of their charges, and knows how to test for and adjust the aquatic environment.


All new and moved livestock is suspect and should be treated prophylactically by dipping, treatment and quarantine. Fish from a reputable dealer and ones that have co-mingled in a show are no doubt infested and must be treated as detailed below. Many parasite problems are perennial with a change in season, brought in with foods, water and birds (yes, that's right, "aerially-borne"). These should be anticipated.

Identification of Parasites:

Is mainly a matter of observing irregular, abnormal behavior such as excessive scratching, glancing, jumping, listlessness, poor feeding; further with observation of sores, bleeding or in advanced stages with larger parasites, seeing the adult forms themselves.

Application of DTHP:

The published values for concentration and application intervals are highly variable. DTHP is sometimes used as a dip; but for almost all circumstances it is far better, safer and effective to utilize the following method.

1) Clean your system as much as practical of organic debris before starting chemical treatments. If your fish(es) are not so debilitated that they can withstand a very large change, dump the system entirely, removing all "sludge". Disinfection guidelines are available in the article pieces on Frequent Partial Water Changes, Treating Tapwater and "Acid & Bleach Washing".

2) Examine all your fish(es) individually. Remove adult parasites (for anchor worm, fish lice) carefully with forceps/tweezers; spot treat the area of removal with a mercury-based compound (mercurochrome, Merthiolate, Merbromin) or equivalent. This is necessary because the intermediate stages of many external parasites are easily killed with low concentrations of DTHP, but attached adults require higher concentrations, and longer exposure.

3) If possible, dip the all the processed individual fish in a dilute Formalin bath (one ounce of formalin or formaldehyde in five gallons of water) for two minutes. Some people still treat external parasites with just formalin, even placing it into their holding system. This is not generally suggested due to narrow safety margin and probability of interrupting biological filtration.

4) Treat the holding pond with one level teaspoon of DTHP (80 % wettable powder) per two thousand gallons of the system. This amount of material should be re-administered three times every four days. Over a possible range of temperatures and pH's, concentrations of biomass among other mitigating circumstances, this treatment will result in a safe, effective cure. Many references cite higher and lower values/dosages.

The source of the DTHP may be diluted with a carrier, have lost effectiveness due to age and exposure. Considering all these factors, something around 0.25 part per million active ingredient over a period of two weeks will kill most of the commonly encountered external parasites of pond fish. Particularly severe, persistent infestations may require an additional series of treatments. If you are up to calculating the actual ppm for your system, be sure to take into account the initial concentration of DTHP in your source; most are 50 or 80%.

Dissolve the DTHP powder in pond water and distribute evenly around the edge of the system. Keep the water in motion through your normal filtration/recirculation system.

There is unfortunately, no inexpensive chemical, physical assay for testing the concentration of DTHP while its in your system. Simple calculations, a gram-balance, more frequent administered, smaller amounts, may give you some assurance that you have "enough" active ingredient in the system during the treatment time.

Administering DTHP With Other Treatments:

In general mixing DTHP with formalin, copper, malachite, salts and other medications is not advised, is unnecessary and dangerous. If your livestock is weak, you may push them over the edge by using more harsh chemicals.

Other Treatment Cautions:

As mentioned above, if you have some snails or other non-fish/invertebrate life in the treated system you'd like to keep, you will have to remove them; returning them only after the parasite problem is cured and the DTHP removed by time, massive water change or carbon filtration. DTHP is notably toxic to waterfowl also.

As with all organo-phosphate pesticides, you should strive to have minimum personal physical contact. Keep your source securely stored away from animals and children, well-labeled, from moisture and heat.

If you have over-treated your fish, you may observe them twitching, of dark color, listless with clamped fins, with abnormal breathing. If this occurs, you should make a large water change immediately or move the affected stock to new water.

One last parting comment; it is becoming more and more common to hear people talk of resistance of parasites to DTHP and concurrent ineffectuality of treatment. Preventative treatment of

your whole system during the Spring and Fall is recommended by some writers, or anytime fish show any abnormal symptoms. I encourage more limited, periodic use of DTHP in conjunction with routine water quality maintenance and quarantine.

Bibliography/Further Reading:

Adachi, Takemi. 1985. Unpublished "Koi Pond Book"

Blasiola, G.C. 1976. "Ectoparasitic Turbellaria," Marine Aquarist, Vol 7, No. 2, pp. 53-58

Herwig, Nelson, 1979. Handbook of Drugs And Chemicals Used In The Treatment Of Fish Diseases, Charles Thomas Publisher, Springfield, Ill.

Johnson, Erik L. 1993. What really kills fishes (water quality problems, over-medication, parasites). TFH 4/93.

Aquatic Gardens

Ponds, Streams, Waterfalls & Fountains:
Volume 1. Design & Construction
Volume 2. Maintenance, Stocking, Examples

V. 1 Print and eBook on Amazon
V. 2 Print and eBook on Amazon 

by Robert (Bob) Fenner
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